Liquid crystal display devices have increasingly replaced cathode-ray tube (CRT) display devices. Such liquid crystal display devices have advantages in features such as energy saving, reduced thickness, lightweight, and the like. For their advantages, the liquid crystal display devices have been widely used in liquid crystal display televisions, monitors, mobile phones, and the like. One way to utilize such advantages of the liquid crystal display devices is to improve an illumination device (so-called a backlight) provided behind a liquid crystal display device.
Illumination devices can be generally classified into a side light type (also known as edge light type) device and a direct backlight type device. The side light type device is configured such that light guide elements are provided behind a liquid crystal panel and light sources are provided at lateral edges of the respective light guide elements. In the side light type device, a light source emits light which is then reflected in a light guide element such that the liquid crystal display panel is irradiated with the light indirectly and uniformly. This configuration makes it possible to realize an illumination device having a reduced thickness and good luminance uniformity although having a low luminance. Thus, the side light type device is mainly used in a medium-to-small-size liquid crystal display of a mobile phone or a laptop personal computer.
One example of the side light type devices is disclosed in Patent Literature 1. Patent Literature 1 discloses a surface-emitting device in which a light guide plate has a reflecting surface provided with a plurality of dots so as to emit uniform light therefrom. In the surface-emitting device, the light is hardly transmitted to a corner of the reflecting surface due to a directivity of a light source, and thus, the corner of the reflecting surface becomes dark. In order to deal with this, the corner of the reflecting surface is configured with dense dots as comparing with the other area of the reflecting surface.
On the other hand, the direct backlight type device is configured such that a plurality of light sources are arranged behind a liquid crystal display panel so as to directly illuminate the liquid crystal display panel. This makes it easier for even a large screen to have a high luminance. Therefore, the direct backlight type device is mainly used in a large-size liquid crystal display of 20 inches or more. However, since currently available direct backlight type device has a thickness in a range of approximately 20 to 40 mm, this results in a barrier to a further reduction in thickness of a display.
A further reduction in thickness of a large-size liquid crystal display can be attained by placing light sources and a liquid crystal display panel closer to each other. In this case, however, it is impossible for an illumination device to have luminance uniformity unless a larger number of light sources are provided. Such an increase in the number of light sources leads to higher cost. In such circumstances, there is a demand for development of a thin illumination device in which good luminance uniformity can be obtained without providing an increased number of light sources.
Conventionally, the following attempt has been made to solve the problem. Specifically, a plurality of the illumination devices of a side light type are arranged for a reduction in thickness of a large-size liquid crystal display.
For example, Patent Literature 2 discloses a surface light-source device having a tandem structure in which plate-like light guide blocks are arranged in tandem and provided with respective light sources each for supplying primary light to corresponding one of the plate-like light guide blocks. In the surface light-source device thus structured, a wide light-emitting area can be secured by a compact configuration. Thus, the surface-light source device as disclosed in Patent Literature 2 is suitably applicable in a large-size liquid crystal display.
Patent Literature 3 discloses an illumination device including an array of light-emitting modules each having a polygonal shape. Each of the light-emitting modules includes a light source, such as LED or the like, at an end of a light emitter.
Such an illumination device as described above, configured by arranging a plurality of light-emitting units each formed by a combination of a light source and a light guide element, is called a tandem illumination device.
Regarding such a tandem illumination device as described above, it is known that each light guide element has a light guide section, in addition to a light-emitting section having a light-emitting surface, so that more uniform light can be emitted entirely from a light-emitting surface of the light guide element, and that the light guide section sufficiently diffuses light emitted from a light source into the light guide element and at the same time guides the light toward the light-emitting section (see Patent Literature 4 below).
Therefore, typical tandem light guide elements are arranged so that a light-emitting section of one of any adjacent light guide elements is on a light guide section of the other light guide element. Thus, in this structure, the light guide elements are arranged so that their respective light-emitting surfaces are continuously flush with each other.